1. Field of the Invention
The present invention relates generally to methods and apparatus for removing baseline wander from an ECG signal and more particularly to such methods and apparatus which utilize a filter having a nonlinear phase response.
2. Description of the Related Art
In the collection and analysis of ECG waveforms, electrodes are connected to a patient for detecting the ECG signal. Sometimes the signal is provided to a Holter monitor which is a recording device worn by the patient. After data is collected for a predetermined length of time, it is transferred to a system which analyzes and displays the collected data. Such a system is suitable for diagnosing certain types of heart conditions. In some situations, however, it is necessary and desirable for a clinician to observe the ECG waveform on a real-time basis. For example, when a patient is being monitored during a stress test it is critical that a clinician observe the ECG waveform substantially as it occurs in case changes in the waveform indicate a condition which would require the test to be varied or stopped altogether.
The baseline of an ECG waveform can vary considerably. Such variations are known as baseline wander. Baseline wander can be caused by, e.g., respiration, poor electrode contact and sweating. There are both high and low amplitude wanders across a range of frequencies. Because the spectra of baseline wander and the ECG signal are very close, and in some cases may overlap, it is difficult to eliminate the wander and leave the ECG signal undistorted, especially when real-time monitoring is required.
There is a prior art technique for removing baseline wander known as the cubic spline method. Using this method, baseline wander is estimated with a third order polynomial and the estimate is subtracted from the ECG signal. This method has a number of disadvantages. To generate the estimate, three data points on an ECG waveform, called knots, are located. The knots are typically located on the PR segments of three consecutive heartbeats in an ECG waveform. Beat times generated by a QRS detector are used to locate the PR segments. Because all QRS detectors sometimes produce false beat times, the ECG is distorted when a data point is derived from a false beat. In addition, in the presence of high noise, the QRS detector may not function. Also, baseline wander with sharp transitions may not be accurately described by a cubic polynomial. The processing delay between the real-time ECG waveform and the corrected waveform varies dependent upon heart rate. The inherent delay is three beats since information from three consecutive beats is needed before the baseline estimate between the first two can be computed. For example, at a heart rate of 60 beats per minute, the delay is at least three seconds.
In addition to the cubic spline method, there are prior art filtering techniques for removing baseline wander from an ECG waveform. Two filters which have been used to filter ECG waveforms include a finite-impulse response (FIR) filter and an infinite-impulse response (IIR) filter. Both FIR and IIR filters can be described by equations having a preselected number of variables and corresponding coefficients which together define the operating characteristics of the filter. The more filter coefficients, the more computations are required to produce a single output data point.
The American Heart Association (AHA) and the Association for Advancement of Medical Instrumentation (AAMI) now suggest 0.67 Hz as the cut-off point of a high-pass baseline wander filter. Any filter for removing baseline wander from an ECG waveform must have a sharp transition region. FIR filters are disadvantageous because many coefficients are necessary to achieve a sharp transition region.
IIR filters, on the other hand, can achieve a sharp transition region with a small number of coefficients. However, an IIR filter which has a cut-off frequency high enough to remove baseline wander has a nonlinear phase response which distorts meaningful components of the ECG waveform. In one paper, David W. Mortara, Digital Filters for ECG Signals, Computers in Cardiology, Pages 511-514 (IEEE, 1977), an IIR filter is described which has a high cut-off frequency which therefore causes less low frequency distortion. This filter, however, has gain in the pass band. Thus, any baseline wander having a frequency which falls in the pass band may be emphasized.
In Erik W. Pottala, et al., Suppression of Baseline Wander in the ECG Using a Bilinearly Transform, Null-Phase Filter, Journal of Electrocardiology 22 (Suppl):244(1989), a filter having a nonlinear phase response is used to filter an ECG waveform. This filter filters the data both forward and backward in time thereby removing nonlinearities injected by the filter. The problem with utilizing this technique is that the filtered signal is not available until after it has been filtered both forward and backward. Thus, prior art filters utilize forward and reverse-time filtering only in the context of removing baseline wander from ECG by running a predetermined length of data, e.g., five seconds worth, first in a forward direction through a filter and thereafter in a reverse-time direction thereby removing baseline wander. Such filtering is not suitable for situations in which near real-time monitoring is required.